It is well known that the properties of earth are such that low frequency electromagnetic waves penetrate farther into the earth than high frequency electromagnetic waves. Consequently, frequencies above 20MHz are of little interest for prospecting in the earth at great depths by the method of transmitting electromagnetic wave energy into the ground and detecting the wave energy reflected from impedance discontinuities in the ground. In such geophysical exploration it is necessary to have an antenna that can easily be moved over the ground to enable the area of interest to be surveyed. Because low frequency antennas tend to be physically large, reduction in size of the antenna is an important consideration for an antenna intended for actual field use. A conventional dipole antenna, as is well known, should be about one half wavelength long. Thus, a conventional dipole antenna for transmitting a 10MHz signal would be about 15 meters in length. Obviously, an antenna that is 15 meters long presents problems in transporting it in the field.
A time domain radar system used for subsurface exploration imposes more stringent requirements on its transmitting and receiving antennas then a narrow band radar system. Time domain radar systems for geophysical exploration require a wide band antenna with a bandwidth in the order of two or three octaves. For example a time domain radar system having a 10MHz center frequency requires its antenna to transmit signals in the band from about 5MHz to about 15MHz. An important consideration for such a broad band transmitting antenna is that the impulse energy travelling along the antenna be nearly completely absorbed when it reaches the outer ends of the antenna to prevent the wave energy from returning back along the antenna and radiating a second signal which masks the reflections of the first signal.